[abis_nm] Add abis_nm_bs11_infield_logon to logon as user field

As this user you are able to set the PLL work value which is especially
useful if your BS11 got detuned by an inaccurate oscillator in your E1
card.
diff --git a/openbsc/src/abis_nm.c b/openbsc/src/abis_nm.c
index bfc3de2..bcfce68 100644
--- a/openbsc/src/abis_nm.c
+++ b/openbsc/src/abis_nm.c
@@ -2423,6 +2423,11 @@
 	return abis_nm_bs11_logon(bts, 0x02, "FACTORY", on);
 }
 
+int abis_nm_bs11_infield_logon(struct gsm_bts *bts, int on)
+{
+	return abis_nm_bs11_logon(bts, 0x03, "FIELD  ", on);
+}
+
 int abis_nm_bs11_logon(struct gsm_bts *bts, u_int8_t level, const char *name, int on)
 {
 	struct abis_om_hdr *oh;